Uplink pilot reuse and user-proximity detection in wireless networks

ABSTRACT

A method and apparatus is disclosed herein for uplink pilot reuse and user-proximity detection in wireless networks. In one embodiment, a wireless system is disclosed for use in a wireless network having a plurality of user terminals (UTs), where at least two UTs of the plurality of UTs have pre-assigned user uplink pilot codes and are allocated common pilot resources over at least two concurrent slots, the system comprising a UT-proximity detection processor to determine which UT channels the UT-proximity detection processor can resolve among a group of UTs transmitting pilots in identical pilot resources, the UT-proximity detection processor operable to resolve a UT channel of a UT by determining the UT is in proximity of the system and no other UT in the group of UTs transmitting pilots that has transmitted a pilot on the identical pilot resources is in proximity of the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a U.S. National Phase application under 35U.S.C. § 371 of International Application No. PCT/US2016/057192, filedOct. 14, 2016, entitled UPLINK PILOT REUSE AND USER-PROXIMITY DETECTIONIN WIRELESS NETWORKS, which claims priority to and incorporates byreference the corresponding U.S. provisional patent application No.62/242,237, entitled, “Method and Apparatus for Aggressive Uplink PilotReuse and Fast User-Proximity Detection in Dense Wireless Networks,”filed on Oct. 15, 2015.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the field of wirelesscommunication; more particularly, embodiments of the present inventionrelate to pilot reuse and proximity detection in wireless networks.

BACKGROUND OF THE INVENTION

Dense cellular network deployments relying on the use of Massive MIMOtechnology are becoming very attractive candidates for future radioaccess technologies. This is partly due to the promise of Massive MIMOfor providing very large throughput increases per BS, due to its abilityto multiplex a large number of high-rate streams over each transmissionresource element.

It is well accepted by now that major gains in the PHY layer in terms ofthroughput per unit area are to come from the judicious use of denseinfrastructure antenna deployments, comprising of a dense network ofsmall cells, possibly equipped with large antenna arrays. Indeed,Massive MIMO is very attractive when it is used over dense (small cell)deployments, as, in principle, it can translate to massive throughputincreases per unit area with respect to existing deployments.

Massive MIMO is also envisioned as a candidate for addressing largevariations in user load, including effectively serving user-traffichotspots spots, such as e.g., malls or overcrowded squares. A deploymentoption that is considered attractive (especially) for servinguser-traffic hots involves remote radio-head (RRH) systems in which a BScontrols a massive set of antennas that are distributed over manylocations. Current proposals for RRH systems consider only one or atmost a few antennas per RRH unit. However, with bandwidth expected tobecome available at higher frequency bands (including in the mmWaveband), it will become possible to space antenna elements far closer toone another and consider RRHs with possibly a large number of antennasper RRH unit. In principle, this would allow the network tosimultaneously harvest densification and large-antenna array benefitsthereby delivering large spectral efficiencies per unit area.

Channel state information (CSI) between each BS antenna and the userterminals is required in order to be able to serve multiple streams overthe same transmission resources. CSI is obtained by the use of trainingpilots. A pilot is transmitted by one antenna and received by another inorder to learn the channel between the two antennas. With massive arraysat the BS side, the preferred option for training (in terms of itstraining overhead) is to train in the uplink, as one pilot from a userterminal (UT) antenna trains all the antennas at nearby BS sites, nomatter how many sites and antennas per site. This is true not only fortransmitting data in the uplink but also for downlink transmission. Byusing UL training and exploiting uplink-downlink radio channelreciprocity, “Massive MIMO” rates can be achieved in the DL, provided ULtraining and DL massive MIMO data transmission are within the coherencetime and bandwidth of the wireless propagation channel.

Furthermore, reciprocity based training inherently enables coordinatedmultipoint (CoMP) transmission, including RRH-based transmission.Indeed, inherently, a single pilot broadcast from a user terminalantenna trains all the antennas at all nearby BS sites that it can bereceived at sufficiently high power. It is well known that in cellularnetworks such CoMP transmission is beneficial for users at the celledge, i.e., for users that receive equally strong signals from more thanone BS. Similar performance gains are expected in RRH systems.Inherently, a user can obtain beamforming gains during the datatransmission phase from all the RRH unit-antenna combinations thatreceive the user's pilot broadcast at sufficiently high power.

An important challenge that arises in harvesting densification benefitswith cellular networks arises from the fact that UL pilot resources mustbe reused over the network. It is desirable to make the reuse distanceof a pilot resource as small as possible in order to maximize thedensification benefits and the delivered network spectral efficiency(and throughput) per unit area. Indeed, if the same pilot resource couldbe effectively reused by two close-by users, this would allow servingthese two close-by users in parallel by the network. However, the userswould have to be significantly separated (geographically), so that theirsimultaneously broadcasted pilots are received by their servingbase-stations at sufficiently high powers, but at sufficiently lowpowers at each other user's BSs. This implies that there is a minimumreuse distance for a users' UL pilot that has to be honored so thatusers using the same pilot have to be significantly geographicallyseparated to not cause interference to each other's BS.

A similar issue limits the throughputs per unit area achievable byRRH's. Indeed, it is conventionally assumed that a pilot resource isused by a single RRH (active) user. This limits the possiblemultiplexing gains offered by the RRH to serving a single user.

To achieve large cell throughputs and (especially) large cell-edgethroughputs over well-planned macro-cellular networks with simplifiedscheduling and precoding operation, it is advocated that a reuse-7operation is used. It is easy to show that in such a massive MIMOnetwork, the advocated operation is effectively equivalent to a reuse-1operation with pilot-reuse 7, whereby the pilots are split into 7subsets and each subset is reused every 7-th cell.

There has been a proposal for a pilot reuse extension of this approachover heterogeneous networks comprising of well-planned macro-cells andsmall cells. In particular, pilot dimensions are split between macrosand small cells. Furthermore the individual tier pilot resources arereused with a given pilot reuse factor. For example, the small cell BSsare colored with a finite set of colors, so that no small cell has asame-color neighbor. The pilot reuse factor in this case corresponds tothe number of used colors. Although, in theory this results in a minimumpilot-reuse of 4, as the minimum number of needed colors is 4, inpractice, larger number of colors (and thus larger pilot reuse factors)are required.

A geographic scheduling approach has been discussed, according to which,in each scheduling slot users at similar locations (relative to theirserving cell) are scheduled for transmission across the network. Thisallows optimizing the precoder, multiplexing gains and the pilot reuseindependently per geographic location, i.e., independently forcell-center and cell-edge users. With this operation, substantial gainswith respect in terms of cell and cell-edge throughput (as well as interms of the number of antennas needed to achieve a certain level ofperformance). However, this approach relies on a well-plannedmacro-cellular network with dense user traffic so that geographicscheduling and optimization are possible. As a result this approachcannot be directly used in unplanned small cell deployments.

Clearly, as higher band frequencies become available and wirelessnetwork become increasingly densified, there is a need for methods thatallow translating antenna/site-densification into gains inspectral-efficiency per unit area. Although, for a well-plannedmacro-cellular network, this may be achieved (in this case the antennasites remain fixed and the number of antennas per site is increased),achieving similar gains with network densification (e.g., in cases whereboth the number of antennas/site and the number of sites also increase)is not possible with the current state of the art methods.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed herein for uplink pilot reuse anduser-proximity detection in wireless networks. In one embodiment, awireless system is disclosed for use in a wireless network having aplurality of user terminals (UTs), where at least two UTs of theplurality of UTs have pre-assigned user uplink pilot codes and areallocated common pilot resources over at least two concurrent slots, thesystem comprising a UT-proximity detection processor to determine, whichUT channels the UT-proximity detection processor can resolve among agroup of UTs transmitting pilots in identical pilot resources, theUT-proximity detection processor operable to resolve a UT channel of aUT by determining the UT is in proximity of the system and no other UTin the group of UTs transmitting pilots that has transmitted a pilot onthe identical pilot resources is in proximity of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1 illustrates an example of a user-proximity detection pilot codeover a number of concurrent slots (using one port/pilot dimension perslot).

FIG. 2 illustrates one embodiment of a RRH system serving users on the1st dimension of a set of concurrent pilots using the code of FIG. 1.

FIG. 3 illustrates a connectivity graph for the example in FIG. 2.

FIG. 4 illustrates a user-packet serving graph for the example in FIG.2.

FIG. 5 illustrates an example of a user-proximity detection code over anumber of concurrent slots.

FIG. 6 is a block diagram of one embodiment of a RRH site system.

FIG. 7 is a flow diagram of one embodiment of a process for performingactive-user proximity detection at a RRH site.

FIG. 8 illustrates a table showing the maximum number of users that canbe supported over the same pilot dimension as a function of the numberof concurrent slots over which the same set of users is scheduled andthe number of slots over which a user broadcasts no pilot.

FIG. 9A illustrates a wireless network having multiple RRHs thatcommunicate wirelessly with UTs.

FIG. 9B illustrate allocation of resource blocks for uplink training anddownlink transmission.

FIG. 10 illustrates one embodiment of the shared pilot dimensions usedby a pair of user terminals.

FIGS. 11 and 12 illustrate examples of when only one user terminal isnear a RRH and when two user terminals are near an RRH.

FIG. 13 illustrates the transition from an orthogonal pilot pattern inwhich there are L pilots per user.

FIG. 14 illustrates an example of applying the proximity detectiontechniques to sectorization in order to determine whether or not the twouser channels transmitting pilots on identical pilot dimensions can beresolved on each of the first four sectors of a base station.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Methods and apparatuses that allow for a much more aggressive pilotreuse in cellular, RRH or other wireless systems in way that thisaggressive pilot reuse translates into densification (throughput/unitarea) gains are described. According to one embodiment, many potentiallyclosely located users transmit appropriately coded pilots over the sametransmission resources allocated for uplink pilot transmission. In thecontext of an RRH system, in one embodiment, certain user-specific pilotcodes allow each RRH unit to determine whether the subset oftransmitting active users (over a common uplink-pilot resource set)whose pilots are received at sufficiently high level comprises a singleuser, multiple users or no users. In the description of this invention,we refer to RRH sites and RHH units interchangeably, although, strictlyspeaking there may be multiple RRH units on a single site. In oneembodiment, the RRH unit determines which user channels it can resolveamong a group of users transmitting pilots in identical pilot resources.In one embodiment, the RRH unit resolves a user channel of a user bydetermining the user is in proximity of the system and no other user inthe group of users transmitting pilots that has transmitted a pilot onthe identical pilot resources is in proximity of the system.

In the case that the RRH unit determines that only a single user isreceived at sufficiently high level, the RRH unit also identifies theidentity of the active user and estimates its channel. By having thesame coded packet available for transmission at each RRH unit, and byhaving each individual RRH unit transmit a user packet only when asingle user is identified nearby (based on pilot code based userproximity detection), significant densification benefits can beharvested. Similar benefits can be harvested in dense small celldeployments, by pushing the same coded packet per user to all nearbysmall cell BSs, and having a small cell transmit a user packet only whenthe single-user is identified.

In one embodiment, the disclosed methods and apparatuses allow forincreasing the network spectral efficiency per unit area in denseantenna/antenna-site network deployments. In one embodiment, this isaccomplished by the combined use of appropriately designed pilot codesfor use in the uplink by active (scheduled) user terminals, and use ofmechanisms for fast user detection at each antenna-site by the network.

Note that the terms “user terminal”, “user equipment” and “user” areused interchangeably throughout the specification to refer to a deviceused by an end user to access the services provided by a wirelessnetwork communication system.

Embodiments are disclosed involving remote radio head (RRH) units (orother wireless systems) in which the base-station controlling thetransmission of the RRH units is able to serve a number of userterminals potentially much larger than the number of dimensionsallocated for uplink pilots per scheduling slot. In particular,techniques are disclosed in which many users are assigned to transmittraining pilots over the same transmission resources. By schedulingtransmissions over a set of concurrent resource blocks or slots, and byusing appropriate coding of the pilots transmitted by the userstransmitting over the common pilot resources, the disclosed techniquesallow each RRH unit to detect a user terminal in proximity to it, when asingle user terminal out of those assigned the same pilot resources isin the proximity of the RRH unit. By ensuring that a commonuser-specific slot-specific coded packet is available for transmissionat each RRH unit, an active user is served if at least one RRH unitdetects this user terminal to be the single active user terminal (amongthose active on the same set of pilot resources) in its proximity.

Although the disclosed techniques may be described in the context of asingle RRH system, in which the remote radio heads are distributed overmany different sites, the disclosed techniques can be readily applied inother related scenarios. For instance, they can also be applied oversmall cell networks and can offer significant improvements in networkspectral-efficiency (and throughput) per unit area when serving mobileusers. The simplest such embodiment considers synchronized (and jointlyRF calibrated) small cells that have available a common packet per userper slot. Each BS can transmit the packet when it detects the user asthe only active user in its proximity among the all active users thathave transmitted uplink pilots over the given transmission resources. Inanother embodiment, an additional step of choosing for each detecteduser one of the base stations that detected the user to send the data tothe user is used. This embodiment does not require synchronization orjoint RF calibration of nearby base stations.

Embodiments of the invention have one or more of the followingadvantages with respect to the state-of-the-art network densificationapproaches:

-   -   1) Embodiments of the invention allow a substantial increase in        the spectral efficiency (and throughput) per unit area in hot        spot areas by use of remote radio head (RRH) units, aggressive        pilot reuse and fast (sub ms) user-proximity detection.    -   2) Embodiments of the invention rely on the use of CoMP        transmission, referred to herein as distributed MIMO, that        requires user-data synchronization across different RRH units,        but does not require channel state information exchange between        BSs.    -   3) Embodiments of the invention rely on having users broadcast        coordinated pilots over a set of concurrent resource blocks to        assist in detection of the users in the proximity of a base        station.    -   4) In one embodiment, the RRHs exploit spatial filtering        (generalization of sectorization) together with user proximity        to improve single-user proximity detection.    -   5) Embodiments of the invention allow a substantial increase in        the spectral efficiency (and throughput) per unit area in        serving mobile users by use of a network of large-scale MIMO        small cells. In one embodiment, CoMP transmission, referred to        as distributed MIMO, is used and requires user-data        synchronization across different RRH units, but does not require        channel state information exchange between BSs. In another        embodiment, fast coordination between nearby BSs is used to        choose a single BS for serving a user among the ones that        detected the user in their proximity.    -   6) In one embodiment, the disclosed pilot-code based        user-proximity detection is also combined with a broad range of        channel estimation algorithms based on uplink pilot transmission        to allow enhancing the multiplexing gains achieved in a variety        of wireless transmission scenarios over a broad range of carrier        frequencies. In one embodiment, involving cellular transmission        over mmWave channels, pilot codes can be combined with        pseudorandom pilot allocation over the OFDM plane and compressed        sensing and channel estimation. Disclosed methods yield        significant multiplexing gains in the number of users that can        be served simultaneously by the RRH system, realizing at the        same time compressed-sensing gains in terms of the savings in        the number of training dimensions per user that are required to        estimate the channel of each user in proximity of an RRH unit.

One or more embodiments of the invention include the following:

-   -   1) Protocols for uplink pilot transmission over a set of        concurrent transmission resource blocks (or scheduling slots).    -   2) Mechanisms for immediate detection at each BS or RRH unit of        the event that a single user is in proximity among the users        transmitting pilots over a common pilot dimension over a set of        concurrent resource blocks.    -   3) Mechanisms for immediate user identification at each BS or        RRH unit when a single user has been detected in proximity among        the users transmitting pilots over a common set of pilot        dimensions over a set of concurrent resource blocks.

In the following description, numerous details are set forth to providea more thorough explanation of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

Downlink MU-MIMO and CSI and Rate Calculations for Massive MIMO

The text that follows provides a brief description of the general areaof downlink MU-MIMO, and methods for obtaining the required channelstate information at the transmitter, and rate calculations for MassiveMIMO.

Conventional downlink MU-MIMO schemes have been at the forefront ofinvestigations in the past decade. These schemes promise spectralefficiency increases by using multiple antennas at the base-station andserving multiple users simultaneously without the need for multipleantennas at the user terminals. This is achieved by using knowledge ofthe channel state information (CSI) between each user and thetransmitting base-station. Having CSIT (CSI available at thetransmitter) allows the transmitter to precode the user streams so thateach user terminal (UT) sees only its own stream. Given a base stationwith M transmit antennas, K single-antenna user terminals can be servedsimultaneously, giving roughly a multiplexing gain equal to min(M,K)with respect to a system serving a single terminal. In Massive MIMO, thenumber of antennas serving users is much larger than number of usersbeing served. In downlink Massive MIMO, for instance, many users can beserved at the same time either using linear zero-forced beamforming(LZFBF), or even the simpler Conjugate Beam Forming (CBF), exploitingthe fact that the number of users served is far smaller than the numberof antennas. As the number of antennas gets large, transmission beamsget sharper, thereby achieving the desired received signal level withmuch lower transmitted power levels. Furthermore, with large antennaarrays, the achieved user rates harden, i.e. variance in user rate dueto fast (e.g., Rayleigh) fading becomes effectively negligible.

For the transmitter to achieve this operation reliably the transmitterneeds to have sufficiently accurate CSIT, i.e., the transmitter needs toknow the channels between itself and each of the users sufficientlyaccurately. The techniques used for acquiring CSIT fall into twocategories. The first class employs M pilots (one per base-stationtransmit antenna) in the downlink to allow each user terminal toestimate the channel coefficients between the user-terminal's ownantenna(s) and those of the base-station. This operation provides eachCSI at each receiving user-terminal (CSIR) regarding the channel betweeneach base-station transmit antenna and the user-terminal receiveantennas. The CSIR, i.e., the CSI information available at eachuser-terminal, is then fed back to the transmitter by use of uplinktransmissions to provide CSIT, i.e., CSI at the transmittingbase-station. This class of CSIT acquisition schemes have two overheads:(i) a downlink pilot overhead, which scales linearly with M (the numberof antenna elements at the transmitting base-station); (b) an uplinkfeedback overhead, responsible for making available to the base-stationthe channels between each user-terminal and each base-station antenna.In the case each user terminal has a single antenna, the uplink feedbackis responsible for providing to the base-station the MK channelcoefficients (complex-scalar numbers), one coefficient for each channelbetween each user terminal antenna and each base-station antenna.Although the uplink overhead could in principle be made to grow linearlywith min(M,K), with the methods used in practice this overhead grows asthe product of M and K. The downlink overhead limits the size of theantenna array, M, that can be deployed. Similarly, the uplink overheadslimit both M and K, as the overheads grow very fast with respect toincreasing M and K.

The second class of CSIT acquisition techniques is referred to asreciprocity-based training schemes. They exploit a property of thephysical wireless channel, known as channel reciprocity to enable, undercertain suitably chosen (M,K) pairs, very high-rate transmission withvery efficient CSIT training. In particular, pilots are transmitted inthe uplink by each user (K pilots are needed, but more could be used)and the corresponding pilot observations at the base-station aredirectly used to form the precoder for downlink transmission. If theuplink training and the following downlink data transmission happenclose enough in time and frequency (within the coherence time and thecoherence bandwidth of the channel), then the uplink training providesdirectly the required (downlink channel) CSI at the transmitter, sincethe uplink and the downlink channels at the same time and frequency arethe same. In this class of techniques, the uplink overheads scalelinearly with K, i.e., with the number of user terminals that will beserved simultaneously. These schemes are also typically envisioned asrelying on TDD (Time Division Duplex) in order to allow uplink trainingand downlink transmission within the coherence bandwidth of the userterminal channel with a single transceiver shared for uplink anddownlink data transmission.

One attractive aspect of reciprocity-based training schemes is that onecan keep on increasing the size of the transmit antenna array, M, makingit “Massive”, without incurring any increase in the training overheads.Although with M>K, increasing M does not increase the number ofsimultaneously multiplexed streams, K, (i.e., K streams aresimultaneously transmitted, one to each user), increasing M inducessignificant “beamforming” gains on each stream (which translate tohigher rate per stream), at no additional cost in training.Alternatively, increasing M allows reducing the transmit power requiredto yield a target rate to a user terminal, thereby allowing for greenertransmission schemes.

Reciprocity-Based Massive MU-MIMO

Consider the problem of enabling MU-MIMO transmission from an array of Mtransmit antennas to U single-antenna user terminals. The downlink (DL)channel between the i-th base-station transmitting antenna and the k-thuser terminal is given by{right arrow over (y _(kι))}={right arrow over (h _(kι))}{right arrowover (x _(ι))}+{right arrow over (z _(kι))}where {right arrow over (x_(ι))}, {right arrow over (h_(kι))}, {rightarrow over (y_(kι))}, {right arrow over (z_(kι))}, denote thetransmitted signal from base-station antenna i, the compound DL channelbetween the two antennas, the observation and noise at the receiver ofuser terminal k, respectively. This model is applicable at any resourceblock. In general, the variables in the above equation areresource-block dependent. This dependency is currently ignored in thenotation for convenience though with an abuse of notation, it will beused when time-sharing across various resource blocks are considered.The amplitude and phase shifts introduced by RF-to-baseband conversionhardware (e.g., gain control, filters, mixers, A/D, etc.) at thereceiver of user terminal k as well as the amplitude and phase shiftsintroduced by the baseband-to-RF conversion hardware (e.g., amplifiersfilters, mixers, A/D, etc.) at the transmitter generating the signal tobe transmitted by base-station antenna i are all included in the DLcompound channel.

Similarly the uplink channel between the k-th user terminal and the i-thbase-station antenna is given by

=

+

where

,

,

,

, denote the transmitted signal from user terminal k, the compounduplink (UL) channel between the two antennas, the observation and noiseat the receiver of base-station antenna i, respectively. The amplitudeand phase shifts introduced by RF-to-baseband conversion hardware (e.g.,gain control, filters, mixers, A/D, etc.) at the receiver ofbase-station antenna i as well as the scalar (complex) coefficient

contains the amplitude and phase shifts introduced by the baseband-to-RFconversion hardware (e.g., amplifiers filters, mixers, A/D, etc.) at thetransmitter generating the signal to be transmitted by user terminal kare all included in the compound UL channel.

In the uplink, the following model may be used:

=

+

where

is the vector of dimension K×1 (i.e., K rows by 1 column) comprising theuser symbols on subcarrier n at symbol time t,

is the M×U channel matrix that includes the constant carrier phaseshifts and the frequency-dependent constant in time phase shifts due tothe relative delays between the timing references of the differentterminals,

and

are the received signal vector and noise at the user terminal.

In the downlink, the following model may be used:{right arrow over (y)}={right arrow over (x)}{right arrow over(H)}+{right arrow over (z)}where {right arrow over (x)} is the (row) vector of user symbols onsubcarrier n at symbol time t, {right arrow over (H)} is the U×M channelmatrix that includes the constant carrier phase shifts and thefrequency-dependent constant in time phase shifts due to the relativedelays between the timing references of the different terminals, {rightarrow over (y)} and {right arrow over (z)} are the received signal (row)vector and noise at the user terminals. Other BSs at sufficiently closedistance cause interference as Network MIMO/joint transmission/CoMP orany other interference mitigation techniques are not considered.Interference from the other access points is included in the noise term.

Assuming perfect calibration, the compound UL and DL channels becomereciprocal, so that

={right arrow over (H)}For simplicity, the thermal noise is neglected. In order to estimate thedownlink channel matrix, the U user terminals send a block of U OFDMsymbols, such that the uplink-training phase can be written as

=

+noisewhere

is a scaled unitary matrix. Hence, the base-station can obtain thechannel matrix estimate

=

+noiseIn order to perform downlink beamforming, the compound channel downlinkmatrix {right arrow over (H)} is used. The ZFBF precoding matrix iscalculated asW=Λ ^(1/2)[{right arrow over (H)} ^(H) {right arrow over (H)}]⁻¹ {rightarrow over (H)} ^(H)where Λ is a diagonal matrix with λ_(m)'s as diagonal elements thatimposes on each row of the matrix W, the row normalization ∥w_(m)∥²=1,for all m.

Hence, the LZFBF precoded signal in the downlink with equal power foreach beam also taking account a distance-dependent pathloss model withthe diagonal matrix G, whose diagonal elements are g_(i)'s.

$\begin{matrix}{\overset{\rightarrow}{y} = {{\overset{\rightarrow}{u}p^{1\text{/}2}G^{1\text{/}2}W\overset{\rightarrow}{H}} + \overset{\rightarrow}{z}}} \\{= {{\overset{\rightarrow}{u}p^{1\text{/}2}G^{1\text{/}2}{\Lambda^{1\text{/}2}\left\lbrack {{\overset{\rightarrow}{H}}^{H}\overset{\rightarrow}{H}} \right\rbrack}^{- 1}{\overset{\rightarrow}{H}}^{H}\overset{\rightarrow}{H}} + \overset{\rightarrow}{z}}} \\{= {{\overset{\rightarrow}{u}p^{1\text{/}2}G^{1\text{/}2}\Lambda^{1\text{/}2}} + \overset{\rightarrow}{z}}}\end{matrix}$$\lambda_{k} = \frac{1}{\left\lbrack \left( {{\overset{\rightarrow}{H}}^{H}\overset{\rightarrow}{H}} \right)^{- 1} \right\rbrack_{k,k}}$Notice that the resulting channel matrix is diagonal, provided that S≤M.MU-MIMO User Scheduling

Although there several methods available in the literature forscheduling multi-user MIMO transmissions at the BS, a widely acceptedclass of methods involves scheduling policies which, at any givenscheduling instant at the BS, schedule the subset of users that wouldyield the highest expected weighted sum-rate. Each user's expected ratein each scheduled set for transmission is a function on theinstantaneous channels of all the users in the scheduled set. Indeed,assuming LZFBF transmission as described in the preceding section, atany given resource block the coefficients λ_(k)'s depend on theinstantaneous channel matrix of all users in the scheduling set (servedby LZFBF), and in particular, they can be expressed as

${{\lambda_{k,S}(t)} = \frac{1}{\left\lbrack \left( {{{\overset{\rightarrow}{H}}_{k,S}^{H}(t)}{{\overset{\rightarrow}{H}}_{k,S}(t)}} \right)^{- 1} \right\rbrack_{k,k}}},$where {right arrow over (H)}_(k,S)(t) denote the compound downlinkchannel matrix for UT-k in the user set S at the tth resource block.Clearly, since the choice of the user set S and/or resource block (t)affects λ_(k), the expected user rates are a function of both thescheduling set and the instantaneous channel realization. Fixing thescheduling time instance, and assuming LZFBF transmission, the problemof choosing the subset S which maximize the weighted sum-rate iscombinatorial in the number of antennas, as the number of possiblesubsets, S, that can be considered for scheduling grows exponentiallyfast with the maximum number of users that can be considered for jointscheduling.

Another important factor defining the scheduling assignments that areproduced by the scheduling policy is the method by use of which the“user weights” are chosen at each scheduling instant prior to performingthe weighted sum rate maximization operation. Although many methodsexist for choosing these weights, a widely accepted class of methods(because of their ability to result in nearly optimal performance withrespect to a fairness criterion belonging to a broad class of fairnesscriteria) is one that relies on the use of “virtual queues” to determinethe instantaneous user-weights in the weight-sum rate optimization.

Massive MIMO arrays at the BSs of a cellular network can substantiallysimplify scheduler operation. In sharp contrast to conventional MIMO, inmany cases the rate provided to an active (scheduled) user by itsserving BS does not depend on the other active users in the same celland in nearby cells and can be in fact predicted a priori. Such type ofoperation is exploited over macro-cellular Massive MIMO deployments toobtain large cell and cell-edge throughput gains with respect to theirconventional MIMO counterparts. This rate-hardening approach can beextended to include operation over heterogeneous networks comprising ofmacro and small cells with Massive MIMO arrays. Near optimal associationand load balancing can be achieved with simple user-BS associationmechanisms and rudimentary round-robin schedulers at each base station.

Overview of Embodiments of the Invention

Embodiments of the invention include methods and apparatuses for fast(sub ms) user-proximity detection and identification based on uplinkpilot transmission from a number of users sharing a common set of pilotdimensions over a set of concurrent resource blocks (or schedulingslots). In one embodiment, a pilot dimension refers to the set ofresource elements on the OFDM plane that are made available to a singleuser for transmitting its uplink pilots during a scheduling slot. Theseresources (e.g., frequency and time resources) are a subset of all thepilot resources allocated over a concurrent set of resource blocks totransmit pilots in order for the BS to learn channels between itsantenna array and user terminals over the time-frequency elementsspanned by these resource blocks.

In one embodiment, the techniques described herein can be applied ateach RRH unit of a RRH system serving a hot spot area and allows eachRRH unit to determine which subset of user channels can be reliablyestimated, and subsequently which user streams to transmit, includingthe beam on which to transmit each of these streams. By having availablethe same packet for any given user for potential transmission atdifferent RRH units, each user terminal can be served, provided at leastone of these RRH units is able to estimate the user channel and serveits packet. Embodiments of the invention rely on judiciously choosing:(a) the number of users that use a common set of pilot dimensions acrossa set of concurrent resource blocks; (b) the pilot patterns used bythese users; (c) the mechanism for detecting whether multiple users havecollided at an RRH unit, or a single user is in proximity of the RRHunit; (d) the mechanism to determine the identity of the active user inproximity at the RRH, provided a single user is detected to be inproximity of the RRH unit. As a result, densification can be achieved,in terms of increasing the number of users simultaneously served by theRRH unit on each set of pilot dimensions, translating in gains inspectral efficiencies achieved per unit area.

In one embodiment, the same techniques are applied over a network ofsmall cells with large arrays to achieve large gains in spectralefficiency delivered per unit area to serving mobile users. In oneembodiment, for a given user and resource block (slot), each BS has thesame user-specific packet available for transmission and transmits it ifit detects the associated active user is the only one in proximity amongthe ones transmitting on the same pilot dimension on the given slot. Avariation of this involves fast coordination among nearby BSs thatdetect a given user (as the single user in proximity among the onesusing a common set of pilot dimensions) in order to select a single BSto serve that user (among the BSs that detected the user).

Embodiments of the invention enables an operator of a wireless networkto serve very large numbers of users in a hotspot area via a RRH systemwith many RRH units and antenna elements per site. Embodiments of theinvention allow users using the same pilot resources to be servedsimultaneously by a RRH unit. This allows network densification benefitsto be realized together with large antenna arrays benefits, therebyenabling very large spectral efficiencies (and throughputs) per unitarea. These deployments would allow the network operator tosubstantially increase the provided throughput per unit area and canprovide substantial improvements in user experience over hot spot areas.

Embodiments of this invention include protocols for uplink pilot reuseacross a RRH-based system or a small-cell network, in conjunction withmethods and apparatuses for fast active-user proximity detection at theassociated RRH units or small-cell base stations. Embodiments of theinvention can enable large densification benefits to be realized in theDL of wireless networks. In one embodiment, the disclosed UL pilotprotocols also combine fast active-user proximity reciprocity withacquisition of the user-channels and reciprocity-based DL MIMOtransmission. More specifically, full channel reciprocity applies withineach resource block (RB) (e.g., set of resource elements in time andfrequency) within the coherence time and coherence bandwidth of thechannel, according to which the channel stays constant. If transmittingan UL pilot, measuring the channel, and then transmitting in the DL (andthe UL and DL channels are still the same), the full reciprocity can beexploited. However, if one considers the channels over two differentresource blocks that are at the same time but at different frequencybands, then the UL channel on RB 1 is the same as the DL channel on RB1, and the UL channel on RB 2 is the same as the DL channel on RB 2.However, the UL channels on RBs 1 and 2 are different. At the same time,however, the average channel power of the UL channels on RBs 1 and 2 isthe same. By transmitting an UL pilot on RB1, the UL (and DL) channelpower on RB 1 and RB2 can be learned. Thus, this proximity reciprocityapplies throughout the whole band (i.e., it is not limited within thecoherence bandwidth of the channel).

Although not described in detail, the same UL pilot reuse protocols andthe corresponding “on-the-fly” user proximity detection mechanisms canbe used also for UL data transmission. In one uplink data transmissionembodiment, the RRH (or small cell BS) decodes the packet of thedetected active-user that is in proximity to it (when a single one isdetected in proximity).

Techniques disclosed herein are henceforth described in detail for RRHsystems. Similar methods can be straightforwardly applied to networks ofsmall-cells. Without loss of generality, in one embodiment, thefollowing scenario involves a BS controller, controlling J RRH units,that is serving a user population based on OFDM transmission. OFDMtransmission resources are split into “slots”, or, resource blocks(RBs), with each slot/RB corresponding to a contiguous block of OFDMsubcarriers and symbols. In one embodiment, within each RB or slot, asubset of users across the network are active, i.e., are scheduled fortransmission.

Without loss of generality, in one embodiment, a scheduler operation,according to which the set of active user terminals (i.e., the set ofuser terminals scheduled for data transmission) is the same acrossseveral (potentially all) concurrent slots. In one embodiment, althoughnot necessary, to make the treatment concrete, a block-fading channelmodel is used where the channel coefficients remain constant within eachRB/slot. In one embodiment, within each set of N concurrent slots, theRRH system schedules a subset of K active single antenna users for datatransmission. For purposes herein, the active users and RRH units areindexed by the indices k and j, respectively, with k from the activeuser set

={1, 2, . . . , K} and j in the RRH set. J={1, 2, . . . , J}. In oneembodiment, the RRH unit j has M_(j) antennas and M_(j)>>1.

In one embodiment, the wireless network uses TDD operation withreciprocity-based channel state information (CSI) acquisition, accordingto which each RRH learns the DL channel on a given slot between itsantenna array and an active user terminal based on an UL pilottransmitted by the user terminal within the given slot. In oneembodiment, for convenience that within each slot, Q pilot dimensionsare allocated for UL pilots. In one embodiment, the Q pilot dimensionscorrespond to the Q time-frequency resource elements of the slot. Inanother embodiment, each of the Q pilot dimensions is a code Q pilotresource-elements, with the Q pilot codes being orthogonal to oneanother. In one embodiment, these resource elements are elements of OFDMsymbols preceding the OFDM resource elements used for DL datatransmission. The time-frequency elements on one or more OFDM symbolsimmediately following the OFDM symbols allocated to UL pilots may bededicated to BS processing (and may also allow the system totransmission from UL (pilot) transmissions to DL data transmission).They provide the computation time necessary for the BS controller of theRRH system to process the received pilot transmissions and construct theprecoded DL data signal that is to be transmitted over the remainingresource-elements of the slot that follow.

For comparison, if used as a baseline, the conventional setting,according to which the RRH system schedules a single active user on eachof the Q pilot dimensions for UL pilots, i.e., K=Q. Each RRH unit has acoded packet ready for transmission for each user (the coded packet foruser k is the same at all RRH units). Every RRH antenna in the vicinityof active user k can estimate its DL channel coefficient to user k fromthe uplink pilot transmitted by user k. This enables the training oflarge antenna arrays (e.g., M_(j)>>1) with training overheadproportional to the number of simultaneously served users. In contrastto feedback-based CSI acquisition, it also allows a user terminal totrain multiple nearby BSs without incurring additional trainingoverheads.

In one embodiment, each RRH unit j in the vicinity of user k estimatesthe channel between its antenna array and the k-th active user based onthe received pilot on the k-th pilot dimension. In one embodiment, eachRRH unit uses the Q user-channel estimates to design a MU-MIMO precoder(e.g., CBF, or ZFBF) to serve the Q (or fewer) active users. An RRH unitmay choose, for instance, not to serve users that are sufficiently farfrom the RRH unit (i.e., users whose channels are received at a weakreceive signal power at the RRH unit). Note that the multiplexing gainof this scheme (i.e., the number of user terminals that can be servedsimultaneously) is at most Q.

Embodiments of this invention comprise mechanisms that achievedensification benefits with aggressive pilot reuse. For convenience, theUL pilot dimensions in each slot are enumerated from 1 to Q. In oneembodiment, a common set of active users are scheduled over a set of Nconcurrent slots on the same pilot dimension. In one embodiment, the setof q-th pilot dimensions on all N concurrent slots are assigned to thesame set of L users for pilot transmission. FIG. 1 shows one suchexample involving the first pilot dimension in N=5 concurrent ports,where the user-proximity detection pilot codes are transmitted over N=5concurrent slots (using one port/pilot dimension per slot), allowingL=N=5 users (UTs) to be multiplexed on the same pilot dimension.Referring to FIG. 1, a “1” implies that the user broadcasts a pilot overthe corresponding slot, while a “0” implies that the user remains silenton a given slot. Unlike the conventional system that would assign asingle user on each of these N=5 pilot dimensions, multiple users (L=5in the example) are assigned to transmit pilots over these pilotdimensions. The pilot pattern (code) however differs from one user tothe next. In this example, each user sends a pilot on N−1 out of the Nq-th pilot dimensions (one q-th dimension per slot) and remains silentin the other dimension. In particular, the n-th user for n=1, 2, . . . ,L=N sends a pilot on all slots except slot n. In principle, the maximummultiplexing gain that can be supported across all Q pilot dimension bysuch an operation is (N−1)Q (as opposed to Q for the baseline scheme).

FIG. 2 illustrates an RRH system serving users on the 1st dimension of aset of N=5 concurrent pilots using the code of FIG. 1. This alsoillustrates the densification benefits provided by this code. Referringto FIG. 2, base-station 300-1 operates a set of RRHs 200-1, 200-2, . . ., 200-15. User terminals 200-n (for 1≤n≤5) operate on pilot dimension 1across N=5 concurrent slots and are each assigned a unique codeword(e.g., out of the ones in FIG. 1). FIG. 3 shows a connectivity graphassociated with the scenario in FIG. 2. Referring to FIG. 3, there is anedge between an RRH unit 100-j and a user terminal 200-n in theconnectivity graph in FIG. 3 if and only if user terminal 200-n is inthe proximity of RRH unit 100-j (i.e., the large-scale gain in thechannel between user terminal 200-n and RRH unit 100-j is sufficientlylarge).

Referring back to FIG. 2, consider reception of the pilot transmissionsof user terminals 200-n over the N concurrent slots at RRH 100-1. FromFIG. 2, RRH 100-1 is in the proximity of user terminal 200-1 but not inthe proximity of user terminals 200-2, 200-3, 200-4, and 200-5. As aresult, RRH 100-1 receives non-negligible signal energy on the 1^(st)pilot dimension of all 5 slots except slot 1. Since RRH 100-1 knows thatall the users but user 100-1 transmitted pilots on the 1^(st) pilotdimension of slot 1, it knows that users 200-2, 200-3, 200-4, and 200-5are not in its proximity. Hence only user 200-1 can be in its proximity.Given also that RRH 100-1 receives non-negligible power on the 1stdimension of the remaining four slots, it knows user terminal 200-1 isindeed in its proximity. Because of this, the channel estimate obtainedfrom the pilot reception on the 1^(st) dimension of all slots exceptslot 1 is associated with user terminal 100-1. RRH 100-1 uses that inits precoder design to transmit the 4 packets for user 200-1 (it doesnot of course sent packets for user terminals 200-2, 200-3, 200-4, and200-5) in the 4 slots user terminal 200-1 has transmitted a pilot. Asimilar operation is followed with 100-2 and 100-5. They both identifyuser 200-1 as the only user in their proximity and transmit the same(slot-dependent) packet to user 200-1, on a precoder based on theirchannel estimate from the 1^(st) pilot dimension observation on each ofthe four slots user terminal 200-1 has transmitted a pilot. Similarly,RRH 100-6 and 100-9 (RRH 100-11 100-13 and 100-14) detect user terminal200-2 (user terminal 200-4) as the only one in their vicinitytransmitting UL pilots on the 1^(st) pilot dimension and thus transmitthe associated slot-dependent packets to user 200-2 (user 200-4). Inevery case, the UE in proximity is identified uniquely by the slot forwith the RRH has a low energy return on the 1^(st) pilot dimension. Itis also worth considering the case in which an RRH is in proximity of 2or more users such as, for example, RRH 100-4. Note that, because bothterminal 200-1 and terminal 200-2 are in its vicinity, RRH 100-4observes significant energy return on the first pilot dimension acrossall N concurrent slots, which indicates to the RRH that at least twouser terminals are in its proximity (and therefore the channel estimatedat the RRH is the sum of the channels of these UEs). This is a“pilot-collision” event. All such RRHs that detect such a“pilot-collision” event (i.e., RRH 100-4, 100-7, 100-10, 100-12, 100-15)remain silent on the 1st pilot dimension, i.e., they do not serve any ofthe user terminals 200-1, 200-2, . . . , 200-5.

FIG. 4 shows a serving graph associated with the scenario of FIG. 2.Referring to FIG. 4, there is an edge between an RRH unit 100-j and auser terminal 200-n in the serving graph if and only if user terminal200-n (is in the proximity of and) is served by RRH unit 100-j (i.e.,the large-scale gain in the channel between user terminal 200-n and RRHunit 100-j is sufficiently large, and no other user terminal that isactive on the same set of ports has a large-scale gain in its channel toRRH unit 100-j that is sufficiently large). From the graph, it is clearthat over the given set of concurrent slots, the RRH unit is able tosimultaneously serve 3 user terminals (200-1, 200-2, 200-4) from thefirst group out of the Q pilot dimensions (ports) as opposed to just 1in the baseline RRH unit.

In general, in one embodiment, if L users are scheduled for transmissionon the same pilot dimension of a set of N concurrent slots with anappropriately designed code, then the code allows the RRH to do thefollowing:

-   -   detect collisions, i.e., identify all events where any two or        more out of the L user terminals are in the proximity of the RRH        unit;    -   identify the single-user in proximity, i.e., in any event where        only one user-terminal is in the proximity of the RRH unit, the        RRH unit can identify that indeed only one user terminal is its        proximity, and can furthermore determine which terminal (out of        the L active user terminals on the pilot dimension) it is.

Thus, the RRH unit determines which user channels it can resolve among agroup of users transmitting pilots in identical pilot resources bydetermining the user is in proximity of the system and no other user inthe group of users transmitting pilots that has transmitted a pilot onthe identical pilot resources is in proximity of the system and canidentify user identity when only a single user is determined to be inproximity of the system.

With such an operation, an RRH system can thus serve on each pilotdimension anywhere from 0 to L users simultaneously. In such a system,there is a trade-off between the RRH system coverage area and theaggressiveness of assigning multiple users on the same pilot dimension.Consider for example of a fixed RRH system serving a fixed coveragearea, and consider increasing L (corresponding to assigning pilotdimensions more aggressively) from L=1 onward. Assume also that theactive users on the 1^(st) pilot dimension are randomly chosen acrossthe geographical area by the BS controller of the RRH system. As L isinitially increased from 1 to, for example, 2, immediate multiplexing isrealized assuming a sufficiently large geographical coverage area. Thisis because, as long as for each user terminal there is one RRH whichsees only that user terminal, 2 users can be served. Increasing L keepson yielding multiplexing gain benefits until a point is reached wherethere are so many users assigned on the same pilot dimension that theprobability of collision outweighs the benefits of increasing the valueof L. In general, the value of L can be chosen to maximize the expectedmultiplexing gains, i.e., the product of L times the probability that anactive user is served by at least one RRH unit (this probabilitydecreases with L, the number of active users on the 1^(st) pilotdimension).

Over a generic RB/slot, letting G_(j) denote the M_(j)×K channel matrixbetween the RRH j antennas and the K users, with the associated k-thuser channel given by the k-th column,g _(kj)=[g _(kj,1) ,g _(kj,2) , . . . ,g _(kj,M) _(j) ]^(T).The channel between RRH j and user k can be expressed as g_(kj)=√{squareroot over (β_(kj)/h_(kj))} with the slow-fading scalar β_(kj)characterizing the combined effect of distance based pathloss and thelocation-based shadowing, and the vector h_(kj)=[h_(kj,1), h_(kj,2), . .. , h_(kj,M) _(j) ]^(T) capturing small-scale fading. Without loss ofgenerality, each link experiences independent Rayleigh fading, i.e., theh_(kj,i)'s are i.i.d. C

(0,1) random variables, where C

(0,1) denotes a complex-valued circularly symmetric Gaussian randomvariable with zero mean and unit variance.

Assume users are scheduled for transmission on sets of N concurrentblocks and L=N users are assigned a common (e.g., 1^(st)) pilotdimension as, e.g., described in FIG. 1. In particular, consider thatfor any given k=1, 2, . . . , N, the user terminal with index ktransmits a fixed energy pilot on the 1^(st) pilot dimension of all Nconcurrent slots except slot k. Let y_(j)[n]=[y_(j,1)[n] y_(j,2)[n] . .. y_(i,M) _(j) [n]]^(T) denote the received signal on the first pilotdimension of slot n by the antenna array at RRH unit j. Let ε[n]=Σ_(l=1)^(M) ^(j) |y_(jl)[n]|² denote the average (over the antennas) receivedsignal energy on the first pilot dimension of slot n by the antennaarray at RRH unit j.

For large M_(j), ε[n] approximately satisfiesε[n]≈Σ_(k∉n)β_(kj)+σ²/M_(j). As a result, if none of the users besidesuser k is in proximity, ε[n] would be small, while if at least one useris in the proximity of the RRH unit j (i.e., β_(kj) is significant forat least on user with index k different from n) ε[n] would be large.Consequently, a simple received energy calculation (averaged over theantennas at RRH unit j) on the 1^(st) pilot dimension of each of the Nslots followed by a threshold comparison to determine high (H) vs. low(L) received energy levels suffices for implementing the code in FIG. 1and for using it for fast user-proximity detection and serving by theRRH unit j.

The pilot code in FIG. 1 allows “multiplexing” over N concurrent slotsup to L=N users. The code allows detecting when a single user is inproximity of an RRH unit, and in addition, identifying the user when asingle user is in proximity Since the detected user is only transmittinga pilot on only N−1 out of the N slots, an RRH unit (and thus the RRHsystem) can only serve the user in N−1 out of N slots. This correspondsto a loss in efficiency, as the multiplexing efficiency of serving thatuser is reduced from 1 user to (N−1)/N. Consequently, the code in FIG. 1allows multiplexing up to L=N users on a single pilot dimension and hasefficiency (N−1)/N.

Other codes can be designed that can “multiplex” L>N users over Nconcurrent slots, while still allowing the RRH to determine whether ornot a single user is in proximity along with identifying the single userin proximity. One class of such codes is demonstrated in FIG. 5.Referring to FIG. 5, a user-proximity detection code over N=5 concurrentslots (using one port/pilot dimension per slot) that allows multiplexingL=6>N users over a common pilot dimension (i.e., the same pilotdimension). A “1” implies that the user broadcasts a pilot over thecorresponding slot, while a “0” implies that the user remains silent onthe given slot. The code has efficiency (N−2)/N=⅗. Letting N=N₁+N₂, thiskind of construction allows multiplexing up to L=N₁×N₂ users. In oneembodiment, a user-terminal is given a two-digit ID with the m-th digittaking one of N_(m) values and the N slots are divided into N₁first-digit slots and N₂ second-digit slots. A user terminal with ID “n₁n₂” sends a pilot over all slots except 1st digit slot n₁ and 2^(nd)digit slot n₂. It is easy to verify that if a RRH unit detects low poweron the first pilot dimension of only first-digit slot n₁ and 2^(nd)digit slot n₂, then the user terminal with ID “n₁ n₂” is identified asthe single user terminal in proximity of the RRH unit.

These codes can be readily generalized. Given a number of concurrentslots N satisfying N=Σ_(i=1) ^(m)N_(i) for some set of positive integersN_(i), up to L=Π_(i=1) ^(m) N_(i) user terminals can be multiplexed (anddetected when in proximity in isolation) on the same pilot dimension.The efficiency of such a code is (N−m)/N.

Other code constructions can be used that can improve upon the maximumnumber of codes, L, that can be multiplexed subject to a given N and agiven efficiency (N−m)/N. In one embodiment, each user given an N-bitbinary ID, whereby each user ID comprises m zeros and N−m ones. A usersends a pilot on the n-th slot if and only if its n-th bit in its IDequals 1. It can be readily verified that in the case a single userterminal is present, the RRH will detect a low-energy received pilotsignal on exactly the m slots for which the corresponding user terminaldid not send pilots. Furthermore, collisions are easily identified: theRRH detects a low-energy received pilot signal in less than m slots. Inone embodiment, for a given N and m, this class of codes allowsmultiplexing up to

$L = \begin{pmatrix}N \\m\end{pmatrix}$user terminals. When N is even, the maximum number of user terminalsthat can be multiplexed on the same pilot dimension is given by

$L = \begin{pmatrix}N \\{N\text{/}2}\end{pmatrix}$and has efficiency ½. When N is odd, the maximum number of userterminals that can be multiplexed on the same pilot dimension is givenby

$L = \begin{pmatrix}N \\{\left( {N - 1} \right)\text{/}2}\end{pmatrix}$and has efficiency (N⁻¹+1)/2. To contrast this code against the one inFIG. 5, with N=5, this code allows multiplexing up to L=10, with thesame efficiency of ⅗.

FIG. 8 illustrates a table showing the maximum number of users that canbe supported over the same pilot dimension as a function of N (number ofconcurrent slots over which the same set of users is scheduled) and m(number of slots over which a user broadcasts no pilot). The table inFIG. 8 considers the maximum L supported over N concurrent slots for N=6and N=12, as a function of m, which is the number of slots over whicheach active user terminal is silent. As is evident from the table, forthe same efficiency, scheduling over N=12 concurrent slots allowssupporting orders of magnitude more active users per dimension on thesame pilot set of dimensions than its N=6 counterpart.

Note that in one embodiment there is an optimal L, i.e., there is anoptimal value for the number of active users to schedule on a common setof pilot dimensions by an RRH system. For instance, as L is increasedstarting from L=1 to L=2 the probability that two (at random) scheduledactive users on the same pilot dimensions collide is very small. As aresult, multiplexing gains (serving two active users at a time) areimmediately realized. This increase in multiplexing gains continues withincreasing L values for sufficiently small L values so that collisionprobability is small. At the other extreme of large L values, where thecollision probability is large, the multiplexing gains diminish andbecome zero in the very large L limit. Clearly, there is an optimal L(or optimal range of L values) where the expected multiplexing gains(product of L times the probability that a user can be served by atleast one RRH unit) are maximized. In general, the optimal L valuedepends on the density of the RRH units and the coverage area of the RRHsystem. Also, given an N value, if there is a code that can provide an Lvalue in the optimal required range, the code (or any code) providingthe maximum efficiency can be used. If, however, for the given N value,there is no code that can provide an L value sufficiently large to be inthe optimal range, then the code yielding the maximum multiplexing gainis selected taking into account in each case the loss in efficiencyincurred by the use of the given code.

FIG. 6 shows a block diagram of one embodiment of a module 100-j at RRHunit j. Referring to FIG. 6, RRH module 100-j includes standard modulesfor MIMO wireless transmission. A transmit processor 115-j at RRH 100-jreceives data for one or more active UTs from a data source 110-j, andinformation indicative of the subset of these UTs to schedule fortransmission from a scheduler module 125-j. Transmit processor 115-jprocesses the data for each scheduled UT that is scheduled by schedulermodule 125-j and generates data symbols to all UTs scheduled fortransmission by the RRH module 100-j. In one embodiment, processor 115-jalso receives and processes control information from acontroller/processor 170-j and provide control symbols.

In one embodiment, controller/processor 170-j also generates referencesymbols for one or more reference signals. In one embodiment, a transmit(TX) MIMO processor 120-j performs precoding on the data symbols, thecontrol symbols, and/or the reference symbols for each UT. In oneembodiment, TX MIMO processor 120-j also receives information fromchannel processor 180-j (via controller/processor 170-j) indicative ofthe channel between one or more of these active UTs and RRH antennas, aswell as transmit power allocation information from TX power allocationunit 190-j (again via controller/processor 170-j).

In one embodiment, processor 120-j provides parallel output symbolsstreams to modulators, MODS (130-ja through 130-jt). Each modulator130-j further processes (e.g., convert to analog, amplify, filter, andup-convert) the output sample stream to obtain a downlink signal. Thedownlink data signals from modulators 130-ja through 130-jt may betransmitted via antennas 135-ja through 135-jt, respectively.

In one embodiment, RRH 100-j, the uplink pilot signals from all activeUT's within a set of concurrent slots are received by antennas 135-jathrough 135-jt, and demodulated by demodulators (DEMODs 130-ja through130-jt). The demodulated pilot signals may be used by proximitydetection processor 185-j to determine on which pilot dimensions acrossa set of concurrent slots there is a single active UT in proximitytransmitting uplink pilots. In one embodiment, as described herein,proximity detection processor 185-j determines which UT channels theUT-proximity detection processor 185-j can resolve among a group of UTstransmitting pilots in identical pilot resources. In one embodiment,UT-proximity detection processor 185-j resolves a UT channel of a UT bydetermining the UT is in proximity of the system and no other UT in thegroup of UTs transmitting pilots that has transmitted a pilot on theidentical pilot resources is in proximity of the system.

In one embodiment, proximity detection processor 185-j exchanges controlinformation with the controller/processor 170-j. The detected active UTspilot-dimension pair combinations are then passed viacontroller/processor 170-j to proximity packet scheduler 125-j, whichmay schedule one or more active UTs for DL transmission and possibly ULdata detection over one or more of the set of concurrent slots.

For the purposes of DL data transmission, the detected active UTpilot-dimension pair combinations may also be passed viacontroller/processor 170-j to TX power allocation module 190-j, which inone embodiment allocates power to the DL signals to one or more activeUTs for DL transmission. In one embodiment, the demodulated pilotsignals output by DEMODs 130-ja through 130-jt are also provided tochannel processor 180-j where the uplink channel is estimated andprovided to controller/processor 170-j. In determining the powerallocation to the DL data signals of the scheduled active UTs, in oneembodiment, TX power allocation module 190-j also uses, as input, theoutput of channel processor 180-j. In the case of UL data transmissionwithin the set of concurrent slots, the uplink signals transmitted fromall active UT's are received by antennas 135-ja through 135-jt, anddemodulated by demodulators (DEMODs 130-ja through 130-jt). In oneembodiment, using the UL data detection schedule provided by proximitypacket scheduler 125-j, the demodulated signals are detected by MIMOdetector 140-j and further processed by a receive processor 145-j toobtain decoded data and control information sent by be subset of activeUTs scheduled for data detection.

In one embodiment, receive processor 145-j may receive detected signalsfrom MIMO detector and provides decoded data to a data sink 150-j andcontrol information to the controller/processor 170-j.

FIG. 7 shows a flow diagram for one embodiment of the active-userproximity detection process performed at RRH unit j (proximity detectionprocessor 185-j of RRH module 100-j) on the first pilot dimension of Nconcurrent slots. The process is performed by processing logic that maycomprise hardware (circuitry, dedicated logic, etc.), software (such asis run on a general purpose computer system or a dedicated machine),firmware, or a combination of all three. In one embodiment, the processis performed by the proximity detection processor of a wireless device.

Referring to FIG. 7, initially, module 100-j obtains observations acrossits antenna array on the first pilot dimension of each slot n, for n=1,2, . . . , N (processing block 410-j). Subsequently, module 185-jcomputes the received pilot signal energy across the antenna array ofRRH unit j on the first pilot dimension on each slot n, for each n=1, 2,. . . , N (step 420-j). Subsequently, module 185-j determines whetherthe number of UEs that are in the proximity of RRH j (out of all activeUEs on the first pilot dimension) is equal to 1 or not (processing block430-j). If that number is 1, the UT in proximity is detected (processingblock 440-j) and the RRH serves that UTs packet using the channelestimate obtained in the 1st pilot dimension (processing block 450-j).If, however, no UEs are detected in proximity, or more than one UT aredetected in proximity (pilot collision), RRH module 185-j serves no UTsbased on its user-channel estimate on the first pilot dimension(processing block 460-j).

Another example of the use of the techniques described herein is shownwith reference to FIGS. 9A and 9B. FIG. 9A illustrates a wirelessnetwork having multiple RRHs, or other wireless systems, thatcommunicate wirelessly with UTs. While the wireless systems are shown asremote radio heads (RRHs), in another embodiment, the wireless systemsmay be base stations.

UTs are assigned uplink pilot resources for training. In one embodiment,the same uplink pilot resources may be assigned to more than one user.For example, UTs 910 and 911 may be assigned the same pilot resources.For example, as shown in FIG. 9B, UTs 910 and 911 may be assignedfrequency and time resource blocks 920.

In one embodiment, where both UTs 910 and 911 are active andtransmitting uplink pilots using the frequency and time resource blocks,RRHs 931 and 932 are able to detect a single UT nearby, namely UTs 910and 911, respectively. As there are no collisions occurring betweenpilot transmissions from multiple UTs, RRHs 931 and 932 are able todetect the unique identifier (ID) associated with UTs 910 and 911respectively. Using this information, wireless systems 931 and 932 areable to serve (e.g., send data to) UTs 910 and 911.

On the other hand, RRH 933 is able to detect a collision between theuplink pilots transmitted by UTs 910 and 911. In such a case, wirelesssystem 933 would not serve either of UTs 910 and 911.

FIG. 10 illustrates one embodiment of the shared pilot dimensions usedby a pair of user terminals. Referring to FIG. 10, the resource block inthe upper left corner of the resource elements is used by UT 1 and UT 2.UT 1 and UT 2 have a unique preassigned code (ID) to identifythemselves. For example, the preassigned code identifying UT 1 is01001111, while the preassigned code identifying UT 2 is 10011011. Thus,both UTs are identified by an 8 digit code. Using a preassigned code of8 digits, the code can support up to

$\quad\begin{pmatrix}8 \\3\end{pmatrix}$UTs on these 8 pilot dimensions.

Both UT 1 and UT 2 broadcasts these preassigned codes as part of thepilot uplink pilot transmissions during the upper left corner resourceblocks. If an RRH detects either of the preassigned codes for UT 1 or UT2 but not both, then that RRH can determine that the UT is in itsproximity, the RRH can identify it through its ID, and the RRH then cansubsequently serve data to that UT. However, if an RRH receives energylevels that indicate that both UTs 1 and UT 2 are in proximity of theRRH, then the RRH determines that a collision has occurred and will notserve data to either of the UTs.

Thus, the RRH can detect a pilot collision (instantly without anymessage exchanges), detect that a single UT is in its proximity, detectthe ID of that single UT in its proximity if only an active user ispresent, and determine that no active users in its vicinity for thispilot dimension.

FIGS. 11 and 12 illustrate the two situations involving the example ofhaving one or two UTs near the RRH describe above. Referring to FIG. 11,in the case that only UT 1 is the only UT in proximity of the RRH, thecode of UT 1 is the only code that is contributing to the pilot energyreceived by the RRH. In this case, the RRH sees three L received energylevels and five H energy levels. From the location of the Ls, the RRHcan identify UT 1 is the only UT in proximity. That is, in this case,the three L received energy levels uniquely identify UT 1. In this case,the RRH can serve (e.g., send data to) UT 1.

Referring to FIG. 12, in this case both UT 1 and UT 2 are in proximityof the RRH and are both transmitting uplink pilots. In this case, thereceived pilot energy represents a combined energy level correspondingto the ones and zeroes in the codes of UT 1 and UT 2. Because the RRHreceives the pilot energies that represent the combined energiesassociated with the codes of UT 1 and UT 2, the RRH sees only one Lreceived energy level. Since there are fewer than three Ls, the RRHdetects a collision and determines that both UT 1 and UT 2 are in itsproximity. Based on this, the RRH does not serve either of the UTs onthese eight pilot dimensions.

Note that the example includes two UTs that share common pilotdimensions (e.g., common pilot resources). However, the techniquesdescribed herein are not limited to detecting the proximity of only twoUTs causing a collision and are applicable to an RRH (or other wirelesssystem) detecting the uplink pilot transmissions of more than two UTs.

FIG. 13 illustrates the transition from an orthogonal pilot pattern inwhich there are L pilots per user. In one embodiment, in the example, Lequals 8. This orthogonal pilot pattern can be converted to anon-orthogonal pattern in which the pilots from two (or more) users arecombined. In one case, there are sixteen pilots per group and both UTsare able to transmit using the same pilot dimensions. As shown, both UT1 and UT 2 use the same resource blocks to transmit. However, in orderto provide for accurate channel estimation, the number is used in thenon-orthogonal pattern is greater than the number of pilots per userusing the orthogonal pilot pattern. Therefore, in the example given, thepreassigned codes are assigned to UT 1 and UT 2 and the number of l's isequal to 11 per user (with 5 0's per user), which is greater than the 8pilots per user in the example of the orthogonal pilot pattern.

Embodiments with RRHs with Directional Antennas Per Site

The techniques described herein are applicable to wireless systems withbase stations that employ virtual sector-based processing according towhich, user-channel estimation and data transmission are performed inparallel over non-overlapping angular sectors. Below, a single-cellscenario involving a single base station (BS) with a massive arrayserving multi-antenna terminals in the downlink of a mmWave channel isdescribed. Note that the techniques disclosed herein are not limited tosuch a wireless communication system.

As discussed above, in one embodiment, the uplink training schemes arenon-orthogonal, that is, the schemes allow multiple users to transmitpilots on the same pilot dimension (thereby potentially interfering withone another). Elementary processing allows each sector to determine thesubset of user channels that can be resolved on the sector (withinsignificant pilot contamination) and, thus, the subset of users thatcan be served by the sector. This allows resolving multiple users on thesame pilot dimension at different sectors, thereby increasing theoverall multiplexing gains of the system.

In one embodiment, a combination of non-orthogonal UL training from theuser terminals based on pilot designs such as those described above andsector-based processing and precoding from the base station with a goalto increase aggregate spatial multiplexing gains and user rates. Thechallenge with two UTs transmitting pilots on the same pilot dimensionis pilot contamination. Pilot contamination can substantially limitmassive MIMO performance, as the beam used to send data (and thereforebeamforming) to one UT, also beamforms unintentionally at the other(contaminating) user terminal.

In one embodiment, multiple UTs are scheduled to transmit pilots on thesame pilot dimension, thereby increasing the number of UTssimultaneously transmitting pilots for training. In one embodiment, thepresence of a massive uniform linear array (ULA) at the BS and a form ofpre-sectorization in the AoA domain are exploited. Elementary processingat each sector allows determining the subset of UT channels that can beresolved on the sector, effectively pilot contamination free. Eachsector then serves only the subset of UTs whose channels it can resolve.This allows resolving multiple users on the same pilot dimension atdifferent sectors, thereby increasing the overall multiplexing gains ofthe system. Note that straightforward generalizations of the techniquesdescribed here in the context of ULAs can be applied to massive uniformpatch arrays (UPA) at the BS. A UPA comprises a patch of antennaelements that are uniformly spaced horizontally and vertically andenable the BS to resolve user channels both in azimuth and elevation.Indeed similar pre-sectorization can be applied to resolve user channelsin the Cartesian product of azimuth AoA domain and elevation AoA domain(i.e., for each combination of azimuth AoA and elevation AoA).

DL MU-MIMO Precoding

A number of DL precoding schemes are described below. In one embodiment,wideband scheduling is used, according to which a scheduling slotcomprises Q>1 concurrent fading blocks, and ti denotes the number ofavailable orthogonal pilot dimensions per fading block. Each fadingblock can be viewed as spanning a contiguous set of time-frequencyelements in the OFDM plane that are within the coherence bandwidth andtime of the user channels. Since the fading blocks in a slot areconcurrent (i.e., distinct fading blocks span distinct subbands over thesame set of OFDM symbols), the fading blocks in a slot are indexed usinga fading-block frequency index f∈{1, 2, . . . , Q}. With thisinterpretation g_(s,k)(f) below corresponds to the channel of user k insector s and fading block f:g _(s,k)(f)=F _(s) ^(H) h _(k)(f)where the M×g matrix F_(s) comprises the s-th set of g consecutivecolumns of F. It is worth remarking that, since the entries of g_(k)(f)are uncorrelated (

[g_(k)(f)g_(k) ^(H)(f)]=Λ_(k)), in this way the M×1 channel vectorbetween a single BS and a user is turned into S orthogonal g×1 sectorchannels with uncorrelated entries. It is assumed that L users (out ofthe total of K_(tot) users served by the BS) are scheduled (in roundrobin fashion) per slot by the BS. In the context of the baselineorthogonal training scheme, the BS schedules L=τ UTs per scheduling slotfor UL pilot transmission. Thus τ UTs send orthogonal pilots on eachfading block, one UT/pilot dimension (K=1). In the context of thenon-orthogonal UL training schemes, in one embodiment, the BS schedulesL=K_(τ) UTs per slot for some K>1. Hence, K>1 UT send pilots per pilotdimension. The notation σ_(k) is used to denote the pilot dimension usedby user k, and K_(σ) is used to denote the indices of users assigned topilot dimension a for 1≤σ≤τ.

In one embodiment, DL transmission occurs over a generic slot, and it isassumed without loss of generality that the scheduled UTs have indicesfrom 1 to L. Assuming user k uses the same beam b=b_(k) for UL pilottransmission and as a receive front-end in the downlink MIMO phase, thereceived signal at user k over one channel use within fading block f isgiven byr _(k)=√{square root over (ρ_(d))}x ^(T)(f)h _(k)(f)+n _(k)  (2)

where x is the precoded signal, and where n_(k) represents, for example,independent and identically distributed (IID) noise with n_(k)˜CN (0,1)and ρ_(d) is the downlink SNR. In the MU-MIMO schemes, in oneembodiment, precoding is sector based. In particular, based on ULtraining, each sector resolves the channels of a subset of the L usersand serves them simultaneously. LettingX _(s,k)=1_([γ,∞])(λ _(s,k))  (3)denote whether or not user k is present on sector s andX _(s) ={k;X _(s,k)=1}  (4)denote the set of all users that are present in sector s. In theprecoding schemes of interest, in one embodiment, a user k will beconsidered resolved on sector s (and will be served by a sector s) ifand only if X_(s,k)=1 and there is no other user k′ sharing the samedimension as user k and for which X_(s,k′)=1. Specifically, D_(s,k)denotes whether or not user k's channel can be resolved on sector s:

$\begin{matrix}{D_{s,k} = {X_{s,k}\left\lbrack {\prod\limits_{k^{\prime} \in {K_{\sigma_{k}}\text{\textbackslash}{\{ k\}}}}\;\left( {1 - X_{s,k^{\prime}}} \right)} \right\rbrack}} & (5) \\{and} & \; \\{D_{s} = \left\{ {k;{D_{s,k} = 1}} \right\}} & (6)\end{matrix}$be the subset of present users whose channels are resolvable in sectors. FIG. 14 shows an example, involving two users using a common pilotdimension, a base station and four of its sectors, and two scatterers.As FIG. 14 reveals, user 1 is present in sectors 1 and 2, while user 2is present in sectors 2 and 3. As a result, the channel of user 1 isresolvable in sector 1, the channel of user 2 is resolvable in sector 3,and neither user channel is resolvable in sector 2 or 4.

In general, not all present users are resolvable and thus D_(s)⊆X_(s).Indeed, as inspection of equation (5) reveals if there are two users kand k′ present in sector s (i.e., X_(s,k)=X_(s,k)=1) that use the samepilot dimension (i.e., with σ_(k)=σ_(k′)), then D_(s,k)=D_(s,k′)=0. Thisis consistent with the fact that neither channel can be resolved due tothe pilot collision. The number of sectors that can resolve (and thuswill serve) user k is hence given by A=πr²

$\begin{matrix}{{N_{k} = {\sum\limits_{s = 1}^{S}D_{s,k}}},} & (7)\end{matrix}$while the number of users that are actually served in the slot is givenbyL′=|{k;N _(k)>0}|and, in general, L′≤L.

In one embodiment, a particular form of linear zero-forced beam-formingis used. In one embodiment, all L′ users are given equal power, that is,power ρ_(d)/L′. Furthermore, for any served user k, its power is equallysplit across all sectors that resolved the user's channel. Hence, thek-th user's stream receives power ρ_(d)/(L′N_(k)) from each sector thatserves the user. The precoded vector signal transmitted by the BS isgiven by 1×M vector

${u^{T}(f)} = {\sum\limits_{s = 1}^{S}{{u^{T}(f)}{V_{s}^{H}(f)}F_{s}^{H}}}$where u^(T)=[u₁ u₂ . . . u_(L)] is the information bearing signal withu_(k)˜CN (0,1), andV _(s)(f)=[v _(s,1)(f)v _(s,2)(f) . . . v _(s,L)(f)]denotes the L×g precoder at sector s and fading block f. In particular,v_(s,k) (f)=0 for any k∉D_(s). For any k∉D_(s), v_(s,k)(f) is in thedirection of the unit-norm vector that is zero-forced to all otherresolvable user-sector channel estimates, i.e., to {ĝ_(s,k′)(f);k′∈D_(s)\{k}} where {ĝ_(s,k)}'s denote the estimates of {ĝ_(s,k)}'s.Also ∥v_(s,k)(f)∥²=1/(L′N_(k)). Note that with this type of precoding,V_(s) (f) is invariant to any scalar (and complex) scaling of any of theĝ_(s,k′)(f), for k∈D_(s).

Substituting the expression for x(f) described below in equation (2),and using the fact that h_(k) (f)=F g_(k)(f), the following is obtained:

$r_{k} = {{\sqrt{p_{d}}{\sum\limits_{s = 1}^{S}{{u^{T}(f)}{V_{s}^{H}(f)}g_{s,k}}}} + {n_{k}.}}$(All L users are given equal power, that is, power ρ_(d)/L. Focusing ona single fading block f, the received signal at user k∈{1, . . . , L} isgiven by equation (2), where x^(T)(f) is the precoded vector signal ofsize 1×M transmitted by the BS.x ^(T)(f)=u ^(T)(f)Vwhere u^(T)=[u₁ u₂ . . . u_(L)] is the information bearing signal withu_(k)˜

(0, 1) and V(f)=[v₁(f) v₂(f) . . . v_(L)(f)] denotes the L×_M precoderat fading block f.)Training, Resolvable Channels and Performance Metrics

Orthogonal and non-orthogonal uplink training and its implications onuser channel resolvability are described below. In one embodiment, eachscheduled user k for 1≤k≤L is scheduled to transmit pilots on pilotdimension σ_(k), that is, one of the τ pilot dimensions. Each pilotdimension comprises Q resource elements, one per fading block. Lettingp_(k)=[p_(k)(1) p_(k)(2) . . . p_(k)(Q)]^(T) denote the uplink pilotvector transmitted by user k, with p_(k) (f) denoting the pilot valueused by user k on fading block f.

The received signal by the BS array which is based on the pilotstransmitted by the user set K_(σ) on the pilot dimension a in fadingblock f is given byy _(σ) ^(ul)(f)=√{square root over (P _(p))}Σ_(k∈K) _(σ) h _(k)(f)p_(k)(f)+w _(σ) ^(ul)(f),  (10)where y_(σ) ^(ul) (f) is the received vector of length M, P_(p) is theuplink SNR, and the noise w_(σ) ^(ul) is IID CN (0, I). Thecorresponding s-th sector observations are given by projecting y_(σ)^(ul) (f) onto F_(s)y _(s,σ)(f)=F _(s) ^(H) y _(σ) ^(ul)(f)=√{square root over (P_(p))}Σ_(k∈K) _(σ) p _(k)(f)g _(s,k)(f)+ w _(s,σ) ^(ul),   (11)where w _(s,σ) ^(ul)=F_(s) ^(H)w_(σ) ^(ul)˜CN(0, I), since F_(s)^(H)F_(s)=I.

A. Orthogonal Training

In the orthogonal training setting, user k for 1≤k≤z transmits pilots onthe dedicated pilot dimension σ_(k)=k (i.e., K_(σ)={σ}), and, as aresult, there is no user k′ for which σ_(k)=σ_(k′). Assuming also,without loss of generality, that p_(k)(f)=1, the associated receivedsignal in fading block f by the BS array based on the pilot transmittedby user k on the pilot dimension k (since σ_(k)=k) from equation (10) isgiven byy _(k) ^(ul)(f)=√{square root over (P _(p))}h _(k)(f)+w ^(ul)(f),while the corresponding s-th sector observations are given byy _(s,k)(f)=√{square root over (P _(p))}g _(s,k)(f)+ w _(s,k) ^(ul).

The precoder uses the following estimate of the k-th user'sinstantaneous channel on fading block f and sector s:ĝ _(s,k)(f)=ŷ _(s,k)(f)  (14)Note that this estimate does not make any use of the pilot SNR, and doesnot rely on knowledge of λ _(s,k).

Inspection of equation (5) reveals, that in the orthogonal scheme,X_(s,k)=D_(s,k), as there is no user k′ for which σ_(k)=σ_(k′). That is,in the orthogonal scheme, a sector can resolve the channel of user k ifa user is present in sector s (i.e., λ _(s,k,≥γ)), and thus X_(s)=D_(s).Subsequently, the sector s forms V_(s)(f) for its resolvable user setD_(s) according to (73) and using ĝ_(s,k)(f) from (14) for all k∈D_(s).

Practical detection schemes for detecting the set of user channels thatare resolvable can be readily devised by exploiting the key fact that

[g _(s,k)(f)g _(s,k)^(H)(f)]=diag(λ_((s−1)g+1,k),λ_((s−1)g+2,k),λ_(sg,k))for each fading block f in the slot. Noting also that

$\quad\begin{matrix}{{\mathbb{E}}\left\lbrack {{{{\overset{\_}{y}}_{s,k}(f)}}^{2} = {{\mathbb{E}}\left\lbrack {{{\hat{y}}_{s,k}(f)}^{H}{{\overset{\_}{y}}_{s,k}(f)}} \right\rbrack}} \right.} \\{= {{tr}\left( {{\mathbb{E}}\left\lbrack {{{\overset{\_}{y}}_{s,k}(f)}{{\overset{\_}{y}}_{s,k}(f)}^{H}} \right\rbrack} \right)}} \\{= {{g\left( {{\rho_{p}{\overset{\_}{\lambda}}_{s,k}} + 1} \right)}.}}\end{matrix}$

Simple practical detection schemes can be devised that benefit fromaveraging both over the g beams and the Q tones, e.g.:

${\frac{1}{Q\;\rho_{p}g}{\sum\limits_{f = 1}^{Q}{{{\overset{\_}{y}}_{s,k}(f)}}^{2}}} - {\frac{1}{\rho_{p}}\frac{\overset{{\hat{D}}_{s,{k = 1}}}{>}}{\underset{{\hat{D}}_{s,{k = 0}}}{<}}{\gamma.}}$The notation above is meant to imply that the detector chooses{circumflex over (D)}_(s,k=1) if the left-hand side exceeds the righthand side (i.e., γ), and {circumflex over (D)}_(s,k−0) otherwise. If{circumflex over (D)}_(s,k)=1, then user k's channel on sector s isconsidered to be resolvable by the BS based on received uplink signal.

B. Non-Orthogonal Training

In the non-orthogonal training setting, in one embodiment, K>1 userspilots are aligned to use a single pilot dimension. As a result, thenon-orthogonal scheme splits the L=K_(τ) scheduled users uniformlyacross the K_(σ) sets, so that |K_(σ)|=K for each σ∈{(1, 2, . . . , τ}.Note that, given X_(s), the set of present users in sector s, the numberof detected and served users from sector s, D_(s), is given by equation(6) and in general satisfies D_(s)⊆X_(s). For example, if there is a σfor which multiple users are present in sector s, i.e., |X_(s)∩D_(σ)|>1then D_(s)⊂X_(s). Such situation would correspond to a collision, thatis, two or more users using the same pilot dimension are present insector s, in which case, neither one's channel is resolvable fortransmission.

Given the set of users with resolvable channels in sector s, that is,given D_(s), sector s forms V_(s)(f) according to that described belowand usingĝ _(s,k)(f)= y _(s,σ) _(k) (f)for all k∈D_(s), and where y _(s,σ)(f) is given by (11). (All L usersare given equal power, that is, power ρ_(d)/L. Focusing on a singlefading block f, the received signal at user k∈{1, . . . , L} is given byequation (2), where x^(T)(f) is the precoded vector signal of size 1×Mtransmitted by the BS.x ^(T)(f)=u ^(T)(f)Vwhere u^(T)=[u₁ u₂ . . . u_(L)] is the information bearing signal withu_(k)˜

(0, 1) andV(f)=[v ₁(f)v ₂(f) . . . v _(L)(f)]denotes the L×_M precoder at fading block f.)

Practical detection schemes that detect which user channels areresolvable in a sector can be also readily devised. Noting that

I[∥ y _(s,σ)(f)∥²]=g(P _(p)Σ_(k∈K) _(σ) λ _(s,k) |p _(k)(f)|²+1)),and assuming a sufficiently large number of beams per sector, g, the RHSof (IV-B) can be approximated ∥y _(s,σ)(f)∥². This suggests that asystem of Q linear equations (one per fading block on pilot dimension σ)can be used to obtain {λ _(s,k); k∈K_(σ)}'s. Letting,r _(s,σ)=[∥ y _(s,σ)(1)∥² ∥y _(s,σ)(2)∥² . . . ∥y _(s,σ)(Q)∥²]^(T)there is a system of equationsr _(s,σ) =gP _(p) P _(σ) λ _(s,σ) +g1+noisewhere λ _(s,σ) is a K×1 matrix whose entries comprise the set {λ _(s,k);k∈K_(σ), and where row f of the Q×K matrix P_(σ) contains the associated|p_(k)(f)|² entries. For each k in K_(σ), let also i_(k) denote theindex i for which [λ _(s,σ)]_(i)=λ _(s,k). If Q≥K, the set of {p_(k)(f);k∈K_(σ), 1≤f≤Q} can be chosen a priori so that P_(σ) has full columnrank, and hence the presence of each user can be individually detected.One such simple detector of the presence of user k is given by

${\frac{1}{P_{p}g}e_{i_{k}}^{T}A_{\sigma}r_{s,\sigma}} - {\frac{1}{P_{p}}e_{i_{k}}^{T}\delta_{\sigma}\frac{\overset{{\hat{X}}_{s,{k = 1}}}{>}}{\underset{{\hat{X}}_{s,{k = 0}}}{<}}\gamma}$where A_(σ)=(P_(σ) ^(H)P_(σ))⁻¹P_(σ) ^(H), and δ_(σ)=A_(σ)1 and wheree_(n) ^(T) is the n^(th) column of the K×K identity matrix. Note thatboth A_(σ) and δ_(σ) are independent of the user channels and can becomputed offline. Subsequently user channel resolvability can bedetected by substituting {circumflex over (X)}_(s,m) for X_(s,m) inequation (5):

${\hat{D}}_{s,k} = {{{\hat{X}}_{s,k}\left\lbrack {\prod\limits_{k^{\prime} \in {K_{\sigma_{k}}\text{\textbackslash}{\{ k\}}}}\;\left( {1 - {\hat{X}}_{s,k^{\prime}}} \right)} \right\rbrack}.}$

Note that a variety of codes can be designed, which result in P_(σ)having full column rank. Indeed, P_(σ) codes with full column rank canbe designed with Q≥K even with p_(k) (f) restricted to take {0,1} valuesthat are dense in the number of ones. However, note that that therequirement that P_(σ) be full column rank is not necessary fordetecting the resolvable user channels. Indeed, there are zero-one codesfor cases where Q<K together and simple detection schemes, that isschemes for detecting which user channels are resolvable.

Note that aligning the same set of users on the same set of pilotdimensions across a set of concurrent slots is attractive due to thesimplicity of operation it offers and the fact that it allows “aligning”the users so that their pilots interfere in subsets. Note also thatother embodiments include active-user pilot interference that is alignedin arbitrary ways. One example corresponds to assigning a set of B>1pilot dimensions per resource block to the same set of active users overN ports. Assuming a user terminal with (at least) B transmit antennas, Buplink pilot dimensions allocated to an active user terminal within aslot would allow serving the terminal with up to B streams per slots. Inthis design however the length over which the pilot code can be designedis NB pilot dimensions. For example, consider scheduling L users overB=2 pilot dimensions per slot, over N=6 concurrent slots. Given thatNB=12 pilot dimensions over which the code is defined, from the Table inFIG. 8 it is shown that up to L=66 active users can be supported withefficiency ⅚ over 6 concurrent scheduling slots. Other embodimentswhere, for example, at least one active user is only active on a subsetB′<B of the allocated pilot dimensions over the N slots can beconsidered. Upon proximity detection at each RRH unit (or RRH sectorsite), if a single active UT is detected among those transmitting apilot on the given pilot dimension, then the RRH unit (or RRH sectorsite) can serve the detected user its packet(s).

Finally, the pilot-code based proximity detection mechanisms disclosedherein can be also combined with a broad range of channel estimationalgorithms based on uplink pilot transmission to allow enhancing themultiplexing gains achieved in a variety of wireless transmissionscenarios over a broad range of carrier frequencies. In one embodiment,involving cellular transmission over mmWave channels, pilot codes can becombined with pseudorandom pilot allocation over the OFDM plane andcompressed sensing and channel estimation. In the baseline compressedsensing scenario, a set of users are allocated each a subset ofpseudo-randomly chosen time-frequency elements in the first few OFDMsymbols of a DL training and DL data transmission slot. By exploitingthe fact that channel response of each user (with respect to a given BSsite) is sparse in angle of arrival (AoA), angle of departure (AoD) andmultipath spread, compressed sensing channel estimation algorithms canrealize significant savings in terms of the number of trainingdimensions per user that are required to estimate each user channel. LetD denote the number of pilot dimensions required per user to estimate auser channel within a scheduling slot in such a scenario. With the codesof the type of FIG. 1, using N=D+1 training dimensions allowsmultiplexing up to L=D+1 users on the same set of training dimensions.Upon detecting a single user in proximity of the BS with the pilot-codesand mechanisms disclosed herein, the D (out of the D+1 trainingdimensions) on which the user has transmitted a pilot allow estimatingthe user channel response. Similarly, pilot codes with m zeros and D=N-mones may be used. This requires allows multiplexing up to

$L = \begin{pmatrix}{D + m} \\m\end{pmatrix}$users on the same set of N=D+m training dimensions. The code structureallows not only the detection of the user in proximity (when a singleuser is in proximity), but also estimation of the user channel (based onthe D non-zero pilots transmitted by the user over the D+m trainingdimensions).

Some portions of the detailed descriptions above are presented in termsof algorithms and symbolic representations of operations on data bitswithin a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The present invention also relates to apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; etc.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

We claim:
 1. A wireless system for use in a wireless network having aplurality of user terminals (UTs), wherein at least two UTs of theplurality of UTs have pre-assigned user uplink pilot codes and areallocated common pilot resources over at least two concurrent slots, thesystem comprising: a UT-proximity detection processor coupled to thereceiver to determine which UT channels the UT-proximity detectionprocessor can resolve among a group of UTs transmitting pilots inidentical pilot resources, the UT-proximity detection processor operableto resolve a UT channel of a UT by determining the UT is in proximity ofthe system and no other UT in the group of UTs transmitting pilots thathas transmitted a pilot on the identical pilot resources is in proximityof the system.
 2. The wireless system defined in claim 1 wherein theUT-proximity detection processor is operable to determine an identity ofthe UT determined to be in proximity of the system when no other UT inthe group of UTs transmitting pilots that has transmitted a pilot on theidentical pilot resources is in proximity of the system.
 3. The wirelesssystem defined in claim 1 wherein the pilot resources are time-frequencyelements.
 4. The wireless system defined in claim 1 further comprising ascheduler operable to serve only that subset of UT channels resolved bythe UT-proximity detection processor by scheduling transmissions to a UTassociated with each UT channel in the subset of UT channels.
 5. Thesystem defined in claim 1 wherein at least two UTs share two distinctpilot resources, and at least one of the UTs transmits pilots at twodifferent pilot powers over two of the pilot resources.
 6. The systemdefined in claim 5 wherein one of the two different pilot powers iszero.
 7. The system defined in claim 1 where the UT-proximity detectionprocessor is operable to determine UT proximity using the pre-assigneduser pilot codes and the received signal energy observations across asite antenna array of the system for the identical pilot resources usedfor pilot transmission by the at least two UTs.
 8. The system defined inclaim 7 wherein the UT-proximity detection processor thresholds eachobservation in a set of the received signal energy observations ofuplink pilot transmissions received from UTs to generate an ordered setof data and then compares the ordered set of data to pre-assigned userpilot codes to determine if a match exists.
 9. The system defined inclaim 7 wherein the pre-assigned user pilot codes are non-orthogonalcodes.
 10. The system defined in claim 7 wherein the preassigned codescomprise a first number of zeros and a second number of ones, where thefirst and second numbers are different, and further wherein theUT-proximity detection processor compares received energy levelsassociated with pilot transmissions with the preassigned codes todetermine if a collision has occurred and the identity of the UT if nocollision has occurred.
 11. The system defined in claim 1 wherein theUT-proximity detection processor is operable to perform proximitydetection where reception of pilot transmissions comprises a pluralityof sector observations from UTs in sectors that are converted into anindication of received pilot energy in a sector, the UT-proximitydetection processor being operable to threshold the received pilotenergy and compare results of thresholding with uplink pilot codes todetermine if a UT is the only active UT in proximity to the wirelesssystem.
 12. The system defined in claim 1 wherein the system is part ofa remote radio head (RRH) site or a base station.
 13. A method foruplink pilot reuse in a wireless network, the method comprising:receiving, at a wireless system, uplink pilot transmissions from one ormore user terminals (UTs) of a plurality of UTs transmitting pilots,wherein at least two UTs in the plurality of UTs are allocated commonpilot resources over at least two concurrent slots; determining, by aUT-proximity detection processor, which UT channels can be resolvedamong a group of UTs transmitting pilots in identical pilot resources bydetermining that a UT is in proximity of the system and no other UT thathas transmitted a pilot on the identical pilot resources is in proximityof the system.
 14. The method defined in claim 13 further comprisingdetermining an identity of the UT determined to be in proximity of thesystem when no other UT in the plurality of UTs transmitting pilots thathas transmitted a pilot on the identical pilot resources is in proximityof the system.
 15. The method defined in claim 13 wherein the pilotresources are time-frequency elements.
 16. The method defined in claim13 further comprising serving only that subset of UT channels resolvedby the UT-proximity detection processor by scheduling transmissions to aUT associated with each UT channel in the subset of UT channels.
 17. Themethod defined in claim 13 further comprising: sharing, by at least twoUTs, two distinct pilot resources; and transmitting, by the at least twoUTs, pilots with different pilot powers over two of the pilot resources.18. The method defined in claim 17 wherein one of the two differentpilot powers is zero.
 19. The method defined in claim 13 furthercomprising detecting, based on received uplink pilot transmissions, atleast two active UTs transmitting pilots on identical pilot resourcesare in proximity of the wireless system, including detecting a collisionbetween uplink pilot transmissions of the at least two active UTs; anddetermining, by the wireless system, not to send data to either of theat least two active UTs in response detecting the at least two activeUTs are in proximity of the wireless system.
 20. The method defined inclaim 13 further comprising scheduling transmissions to UTs resolved bythe system by determining that each of the UTs is in proximity of thesystem and no other UT that has transmitted a pilot on the identicalpilot resources is in proximity of the system.
 21. The method defined inclaim 13 further comprising determining UT proximity using thepre-assigned user pilot codes and the received signal energyobservations across a site antenna array of the system for the identicalpilot resources used for pilot transmission by the at least two UTs. 22.The method defined in claim 21 further comprising thresholding eachobservation in a set of the received signal energy observations ofuplink pilot transmissions received from UTs to generate an ordered setof data and then compares the ordered set of data to pre-assigned userpilot codes to determine if a match exists.
 23. The method defined inclaim 22 wherein the pre-assigned user pilot codes are non-orthogonalcodes.
 24. The method defined in claim 13 wherein the preassigned codescomprise a first number of zeros and a second number of ones, where thefirst and second numbers are different, and further comprising comparingreceived energy levels associated with pilot transmissions with thepreassigned codes to determine if a collision has occurred and anidentity of the UT if no collision has occurred.
 25. The method definedin claim 13 wherein the wireless system comprises a remote radio head(RRH) site or a base station.
 26. A wireless communication systemcomprising: a plurality of UTs, wherein at least two UTs of theplurality of UTs have pre-assigned user uplink pilot codes and areallocated common pilot resources over at least two concurrent slots; aplurality of wireless sites, wherein at least one wireless sitecomprises a UT-proximity detection processor coupled to a receiver todetermine which UT channels the UT-proximity detection processor canresolve among a group of UTs transmitting pilots in identical pilotresources, the UT-proximity detection processor operable to resolve a UTchannel of a UT by determining the UT is in proximity of the system andno other UT in the group of UTs transmitting pilots that has transmitteda pilot on the identical pilot resources is in proximity of the system.27. The wireless communication system defined in claim 26 wherein theUT-proximity detection processor is operable to determine an identity ofthe UT determined to be in proximity of the system when no other UT inthe group of UTs transmitting pilots that has transmitted a pilot on theidentical pilot resources is in proximity of the system.
 28. Thewireless communication system defined in claim 26 wherein the pilotresources are time-frequency elements.
 29. The wireless communicationsystem defined in claim 26 further comprising a scheduler operable toserve only that subset of UT channels resolved by the UT-proximitydetection processor by scheduling transmissions to a UT associated witheach UT channel in the subset of UT channels.
 30. The wirelesscommunication system defined in claim 26 wherein at least two UTs sharetwo distinct pilot resources, and at least one of the UTs transmitspilots at two different pilot powers over two of the pilot resources.31. The wireless communication system defined in claim 26 wherein one ofthe two different pilot powers is zero.